When it's your time it's your time. Like I said it depends on the individual. Unless you are a experienced Judoka, it would be virtually impossible for you to estimate a Judoka's reaction.My recent accident (and my first) was on a paved bike path.
Went airborne and landed on the side of my head in a concrete ditch... helmet saved my life. Landed on my forearm and knee first but did not keep me from banging my head and passing out.
Stuff happened so fast that even a black belt probably could not react in time.
You can see the dent in the picture I posted in the "Helmet... wear them" thread... just a precautionary tale, no one thinks they will get in an accident... but that's why it's called an accident.